As a single crystal for slicing wafers or the like which are used as substrates for semiconductor devices such as memory or CPU, for example, a silicon single crystal can be exemplified and mainly has been produced in accordance with Czochralski Method (Hereinafter, abbreviated as CZ method).
When a single crystal is produced by CZ method, it is produced, for example, by using a single crystal-producing apparatus 1 as shown in FIG. 5.
The single crystal-producing apparatus 1 has members for containing and melting a raw material polycrystal such as silicon and heat insulating members for insulating heat, and they are contained in a main chamber 2. A pulling chamber 3 extending upwardly is continuously provided from a ceiling portion of the main chamber 2, and a mechanism for pulling a single crystal 4 (not shown) with a wire 5 is provided above it.
In the main chamber 2, a quartz crucible 7 for containing a melted raw material melt 6 and a graphite crucible 8 for supporting the quartz crucible 7 are provided, and these crucibles 7 and 8 are supported by a shaft 9 so that they can be rotated and moved upwardly or downwardly by a driving mechanism (not shown). To compensate a depression of a melt surface of the raw material melt 6 caused in connection with pulling of a single crystal 4, the driving mechanism for the crucibles 7 and 8 elevates them as much as the melt level depression.
And, a graphite heater 10 for melting the raw material is provided so as to surround the crucibles 7 and 8. A heat insulating member 11 is provided outside the graphite heater 10 so as to surround it in order to prevent that the heat from the graphite heater 10 is directly radiated on the main chamber 2.
Moreover, an inert gas such as argon gas is introduced into the main chamber 2 from a gas inlet duct 14 provided at an upper part of the pulling chamber 3. The introduced inert gas passes through a space between the single crystal 4 under pulling and a gas flow guide cylinder 12 and a space between a lower portion of the gas flow guide cylinder 12 and the melt surface of the raw-material melt 6, and then is discharged from a gas outlet duct 15.
In addition, a heat shielding member 13 is provided so as to face the raw material melt 6 at a lower edge outside the gas flow guide cylinder 12, and so that radiation from the surface of the raw material melt 6 is cut and so that the heat in the surface of the raw material melt 6 is kept.
A raw polycrystal is contained in a quartz crucible 7 provided in the single crystal-producing apparatus 1 as described above and heated by a graphite heater 10, and thereby the polycrystalline raw material within the quartz crucible 7 is melted. A seed crystal 17 fixed with a seed holder 16 connecting with the lower end of the wire 5 is immersed into the raw material melt 6 into which the polycrystalline raw material is melted as described above, and then the seed crystal 17 is pulled with rotation, and thereby the single crystal 4 having desired diameter and quality is grown under the seed crystal 17. At this time, after the seed crystal 17 is immersed into the material melt 6, a so-called necking that a neck portion is formed by once narrowing the diameter to about 3 mm is performed, and next it is enlarged to a desired diameter and a dislocation-free crystal is pulled.
Usually, in a silicon single crystal pulled as described above, there are intrinsic point defects of vacancy type (Vacancy) and interstitial type (Interstitial). Saturated concentration of the intrinsic point defects is determined by a function of temperature and they become in a supersaturation state with temperature lowering under the crystal growth. In the supersaturation state, annihilation, out-diffusion, up-hill diffusion, and so forth, occur, followed by proceeding in the direction of relaxing the supersaturation state. As a result, any one type of vacancy type and interstitial type remains as dominant supersaturated point defects.
And, it is known that if the crystal growth rate is high, the crystal is easy to be in a state that vacancy-type defects is excess, and conversely, if the crystal growth rate is low, the crystal is easy to be in a state that interstitial-type defects is excess. If the excess concentration becomes a criticality or more, they are aggregated to form secondary defects under the crystal growth.
And, it is known in the case that the growth rate (V) is changed from a high rate to a low rate in the crystal axis direction, a defect distribution map as shown in FIG. 6 can be obtained.
In the case that the growth rate is relatively high rate, vacancy-type defects become dominant in the single crystal. In this case, as secondary defects, Void defects observed as COP (Crystal Originated Particle), FPD (Flow Pattern Defect), or the like are formed. And, a region in which these defects are distributed is called as V region. Moreover, it is known that in the vicinity of the boundary of the V region, defects observed as OSF (Oxidation Induced Stacking Fault) after oxidation treatment are distributed. And, a region in which these defects are distributed is called as OSF region. These secondary defects lead to degradation of oxide film properties.
On the other hand, in the case that the growth rate is relatively low, interstitial-type defects become dominant in the single crystal. In this case, secondary defects observed as LSEPD (Large Secco Etch Pit Defect), LFPD (Large Flow Pattern Defect), and so forth, which are originated from dislocation loops are formed. And, a region in which these defects exist is called as I region. These secondary defects lead to serious fault such as leakage.
In recent years, it is confirmed that between V region and I region and outside OSF region, there exists a region in which there are neither FPD, COP, and so forth originated from vacancies, nor LSEPD, LFPD, and so forth originated from dislocation loops into which interstitial silicones aggregate. The region is called as N region (defect-free region). Moreover, the N region is further categorized that Nv region adjacent outside OSF region (a region in which a large number of vacancies exist) and Ni region adjacent to I region (a region in which a large number of interstitial silicones exist). It is found that in Nv region, when heat treatment is performed, amount of precipitated oxygen is large and in Ni region, oxygen precipitation is little.
With regard to these defects, it is thought that the introduced amount is determined by a parameter of V/G value, which is a ratio of the growth rate (V) and a temperature gradient (G) near the growth interface (See, for example, V. V. Voronkov, Journal of Crystal Growth, 59 (1982) 625-643). That is, if the growth rate (V) and the temperature gradient (G) are adjusted so that the V/G value becomes in a predetermined range, a single crystal can be pulled in a desired defect region.
And, the crystal pulled so that the growth rate (V) and the temperature gradient (G) near the growth interface are adjusted so as to be N region is called as a defect-free crystal. For pulling a defect-free crystal in which a concentration of excess point defects in the single crystal is lowered without limit, it has been required that the growth rate (V) and the temperature gradient (G) are controlled so that a V/G value as indicated by the growth rate (V) and the temperature gradient (G) near the growth interface becomes in a very limited range (See, for example, Japanese Patent Application Laid-open (kokai) No. 8-330316 and No. 11-79889). However, it is very difficult to pull a single crystal by adjusting the growth rate (V) and so forth so as to be in a very narrow range. Therefore, there has been a problem that in the case of pulling a defect-free crystal, defectives practically occur frequently and yield and productivity are largely lowered.